Using both biochemical and genetic tools, we plan: (a) to further explore the regulatory role of the first specific enzyme in leucine biosynthesis, alpha-isopropylmalate synthase, in enteric bacteria; (b) to study leucine pathway regulation in yeast, in order to compare the bacterial system with that of a more highly developed, eukaryotic organism and to find out in what way the regulation of a given metabolic pathway is affected by increasing cellular organization and complexity; (c) to express yeast DNA coding for alpha-isopropylmalate synthase in an apropriate E. coli host (by recombinant DNA techniques) and to study the physiological consequences of the presence of a foreign regulatory enzyme.